Electric control system



Sept. 3, 1946! w. N. GITTINGS ETAL 2,407,072

' ELECTRIC CONTROL SYSTEM 7 Filed 0st,. 24, 1944 v 4 Sheets-Sheet 1 Fig. la.

PHASE SHIF T EH P07E/VTML Inventors: William N. Sittings} Amos W. Bateman,

Thelr" Attorney.

Sept. 3; 5- w. N. GITTINGS ET AL J 2,407,072

ELECTRIC CONTROL SYSTEM Filed Oct. 24, 1944 4 Sheets-Sheet 2 FHA SE SH/F 7' E H Inventors: WiHiam N.Gittings, Amos W. Bate an,

Thai Atto rn ey.

p 1946 w. N. GITTINGS. ETAL 07,072

ELECTRIC CONTROL SYSTEM Filed Oct. 24', 1944] 4 Sheets-Sheet 3 Pi .Sa

Inventor's: William N. Gitt'mgs, ArnOs W. Bateman,

T heir Attorney.

v Sept. 3, 1946. 'w. N. GITTINGS ET AL 2,407,672

ELECTRIC CONTROL SYSTEM Filed Oct. 24, 1944 4 Sheets-Sheet 4 Fi 3b.

Inventors: William N. Sittings, Amos W. Bateman,

Then- Attbrney Patented Sept. 3, 1946 ELECTRIC CONTROL SYSTEM William N. Gittings, Oakmont, and Amos W. Bateman, Yeadon, Pa, assignors to General Electric Company, a corporation of New York Application October 24, 1944, Serial No. 560,161

32 Claims. 1

Our invention relates to electric control systems and more particularly to electric control and regulating systems for power conversion systems.

In order to illustrate our invention, we will describe suitable apparatus for carrying it into effect in an electronic conversion system but it will, of course, be understood that our invention in its broader aspects is not limited to the particular application described but is gene-rally applicable, in whole or in part, to various systemsof control or regulation in other electric systems.

A type of electronic power conversion equipment to which our present invention is applicable is that type sometimes known in the art as a dual conversion system wherein the power from an alternating current supply circuit is rectified by a group of tube rectifiers, transmitted as direct current over a direct current loop to a group of tube inverters which, in turn, deliver power of the same frequency as the sup-ply circuit, or of a different frequency, to an alternating current load circuit. Such apparatus may be used to interconnect two electrical systems: and may be classified as a system tie, a frequency changer, or a direct current transmission system, depending upon the: principal function performed by the conversion apparatus.

In such systems it is usually required that the power flow be reversible at will and that it be possible to adjust the flow of power to any desired value in either direction. Thus, various regular ing and control means including selective control of dynamo-electric machines and switchgear are involved in carrying out our invention.

It is an object of our invention to provide a new and improved electric control and regulating sys- "em.

It is another object of our invention to provide a new and improved regulating system for electronic power conversion apparatus interconnecting two alternating current systems which will enable the power interchanged between such systems to be controlled at any desired value for either direction of power flow.

It is another object of our invention to provide new and improved control and regulating means for dynamoelectric machines.

It is another object of our invention to provide new and improved regulating means in an electric control and regulating system.

It is another object of our invention to provide a new and improved control system for a plurality of electronic power conversion units operated for interchange of power between two alternating current systems.

It is a further object of our invention to provide new and improved fault control and protective means in an electronic conversion system.

Our invention will be better understood from the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the: appended claims.

In the accompanying drawings, Fig. la and Fig. 1b are, respectively, a first and second section of a diagrammatic illustration of one embodiment of our invention in an electronic conversion system of the dual conversion type; Fig. 2 is an explanatory diagram, and Fig. 3a and Fig. 3b are, respectively, a first and second section of a modification of the invention illustrated in Fig. 1a and Fig. lb and illustrates the control of a plurality of electronic conversion units.

Referring now to Figs. 1a and 1b, we have indicated an electronic power conversion system in a simple schematic. form since the particular arrangement and features of the electronic power circuits per se, as distinguished from switching and control circuits, do not form a part of our invention. A satisfactory arrangement of the electronic power circuits for commercial purposes in described and claimed in an application of August Schmidt, Jr., Serial No. 539,939, filed June 12, 1944, and assigned to the assignee of the present application.

In the system as illustrated in Figs. la and 1b, We have shown one alternating current system I connected to be energized by a dynamo-electric machine 2. Such a system may be taken as representative of a public utility cycle system which supplies power to an industrial customer. An industrial load, indicated by the rectangular outline 3, is connected to be energized from a supply circuit 4 which, in turn, is connected through a switch 3 to the system I. It may also be assumed that the industrial customer has local 60 cycle generators, indicated by generator 5, which also feed power to the circuit 4 to assist in supplying power to load 3. A second alternating current circuit 6 is shown at the opposite end of the system with a dynamo-electric generator I and an industrial load 8. For purposes of illustration, the circuit 6 and the connected apparatus may be taken to represent a 25 cycle alternating current system of an industrial customer. The two alternating current systems, namely the assumed 60 cycle system I and the assumed 25 cycle system 6, are interconnected by electronic power conversion equipment, indicated generally by the numeral 9, which is arranged to transfer power in either direction between the two alternating current systems. I and 6.

The electronic power conversion apparatus 9 is interconnected with the. alternating current system I through transforming means IE, bus II and suitable circuit interrupting means I2, and is similarly connected to circuit 6 through transforming means- I3, bus I4 and suitable circuit interrupting means I5.

The electronic conversion apparatus 9 comprises two converter tube groups I6 and I I shown in a very simple form and each comprisin a group of six tubes connected in a three-phase double-way (full wave) circuit. A particular arrangement of transformers and tube groups suitable for a commercial arrangement is shown and described in the above-identified Schmidt application. In the simple arrangement illustrated herein, the tube group I6 comprises siX tubes I8 in which tubes 180 degrees apart in phase po sition are mounted back-to-back with the cathode of the upper tube connected to the anode of the tube below it, as viewed in the drawings. Similarly, the tube group ll comprises six tubes I9 connected in a three-phase double-way circuit in which the tubes 180 degrees apart in phase position are mountedback-to-back. We have illustrated the tubes as being of the ignitron type although our invention is not limited to applications involving tubes of any particular type. Each of the several tubes l 8 of group I6, as illustrated, is provided with an anode 20, a mercury pool cathode 2|, an immersion ignitor or are initiating member 22, a holding anode 23 and a control electrode or grid 24 to determine the instant of conduction in each tube in the proper sequence for rectification or inversion. Similarly, the several tubes IQ of tube group I! are provided with an anode 25, a cathode 26, an immersion-ignitor or are initiating member 21, a holding anode 28 and a control electrode or grid 23. The direct current terminals of the tube groups I6 and I1 are interconnected by a direct current loop circuit 3!! having interposed therein a direct current reactor 3| to suppress voltage ripple in the direct current circuit.

Each of the tube groups is furnished with suitable excitation circuits for each ignitor, holding anode, and control electrode but since the particular form or details of the excitation circuits do not form a part Ofour invention, we have illustrated these circuits in a very schematic form. Thus each tube I8 of conversion unit I6 would be provided with excitation circuits indicated by the cathode-ignitor circuit 32, the holding anode circuit 33 and the grid circuit 34 connected to the several electrodes of the lower right-hand tube I 8 of group I6, as viewed in the drawings. Unidirectional conducting devices 35 and 35' are connected in circuit with ignitors 22 and 21, respectively, in a conventional manner. These circuits are arranged to be energized from suitable excitation means indicated by the rectangular symbol 36 with a notation phase shifter which would include the necessary transformers, reactors and other elements utilized in excitation circuits of this type. The ignitor circuit 32 comprises an output transformer 31 having a pair of secondary windings 38 which are connected, re-

spectively, to tubes [8 displaced 180 electrical degrees. Similarly, the holding anode circuits 33 and grid circuits 34 are Supplied from an output transformer 39 having a pair of secondary windings for the associated pair of tubes. The holding anode circuit 33 is. supplied through an insulating transformer 40 and the grid circuit 34 is supplied through a peaker transformer 4| and includes a bias means 42. An ignitor short-circuiting switch 43 having an operating coil 44 is provided to short-circuit the ignitor transformers for the purpose and in the manner to be described later. A suitable form of ignitor and rid control circuits, in accordance with the schematic arrangement disclosed, is described and claimed in an application of B. D. Bedford, S. No. 539,941, filed June 12, 1944, and assigned to the assignee of the present application. The excitation circuits are arranged to be supplied from a source of alternating current which is correlated in phase and frequency with the alternating current circuit associated with tube group I6. As illustrated, the excitation circuit means 3'6 is connected-to be energized from the circuit 4 through circuit 45 having therein a suitable switch 46 and transformer 41. The device 36 includes a phase shifting means which is controlled in accordance with the differential voltage between an adjustable fixed component of voltage introduced from a resistor 48 and a variable component of voltage depending upon an electrical condition of tube group I6 operating either as a rectifier or an inverter. The variable component of voltage is introduced in the input circuit of device 36 by resistor 49 which is connected by the switch 50 to be energized in accordance with an electrical condition of the tube group I6 operating either as a rectifier or inverter, as indicated by the R and I positions of switch 50. A shift in the phase relation between the various control electrodes and the associated anodes is utilized to obtain the desired power flow between circuits I and 6 in the manner to be described later. A suitable and commercially satisfactory form of phase shifter and control apparatus, in accordance with the schematic form illustrated, is of the type described and claimed in an application of B. D. Bedford, Serial No. 539,942, filed June 12, 1944, and assigned to the assignee of the present application. In the form of phase shifter disclosed in this Bedford application a voltage component variable in accordance with an electrical condition of the tube group [6 or H, such as the current input thereto, constitutes the variable signal voltage and is compared with an adjustable reference voltage which is indicated in the accompanying drawings by the voltage drop across the resistor 48. This reference voltage may be obtained from a well known form of three phase induction regulator 5| connected to be energized from an input circuit 52 and having its usual series and shunt windings connected in a manner to provide an output voltage across its output circuit 53 which is variable in magnitude in accordance with the position of its rotatable element 54. The output circuit 53 is connected to energize a transformer 55 having a primary winding 56 and two secondary windings 51 and 58. The voltage of secondary winding 51 is rectified by a suitable rectifier 59 and impressed across the resistor 48.

Similarly to tube group [6, each tube I9 of tube group I! is provided with ignitor, holding anode and grid control excitation as indicated by the circuits 32, 33 and 34, respectively, leading from the excitation means 36. This excitation means is energized from the bus I4, the alternating current circuit associated with tube group I1, through a transformer 6|]. A switch 43 having an operating coil 44' is arranged for shortcircuiting the ignitor transformer 31' in the same manner as has been illustrated for ignitor transformer 37 of tube group iii. The signal voltage, variable in accordance with an electrical condition of the tube group IT, is introduced into the phase shifter by resistor 49' and switch 56. An adjustable reference component of voltage is derived from secondary winding 58 through rectifier 59' having its direct current terminal connected across resistor 48'.

Although we have illustrated schematically a particular form of means for effecting phase shift by adjusting a reference voltage correlated with a variable component so that phase shift in one direction or the other is effected, depending upon the resultant between the two voltage components, it will be evident to those skilled in the art that our invention in its broader aspects ray be carried out in response to a variable element which effects a phase shift directly or indirectly between two voltages as a result of the movement of a movable element in response to the condition to be regulated.

As fully explained in the aboveidentified Bedford application S. No. 539,942, the desired load on the electronic conversion unit, in accordance with the means herein illustrated, is set by the induction regulator 55 in accordance with the reference voltage obtained across resistor 48 or 48' which is proportional to the rotation of rotor 54. Generally speaking, if unit it is operating as a rectifier, unit I! will then be operating as an inverter. The grids of tubes !8 of the rectifier will then be advanced from a predetermined lagging phase position toward an in-phase or actually to an in-phase position to obtain maximum power flow from circuit l to circuit 6. At the same time, the grids of the inverter tub-es 59 will be operated in an advance position from a predetermined minimum advance position for inverter operation to a greater advance position to maintain the desired commutating angle required for deionization. For reverse power flow, the tube group I! would be similarly operated as a rectifier and the tube group I6 would then be operated as an inverter.

In Fig. 2 we have shown a simple diagram which illustrates, in a general way, the power flow in the conversion system. Rotation of the rotor 54 from A to B or from C to D represents increase in power flow from zero to maximum for either direction of power flow as indicated.

In accordance with our invention, we provide an electric control and regulating means which effects control of the power flow in the electronic conversion system in either direction. In accordance with the illustrated embodiment of Figs. la and lb, we provide a direct current motor (5| for operating the rotor 54 through a suitable shaft 52 and suitable gearing 63 which, in turn, is connected through. a suitable shaft 64 to the rotatable element 54 of the induction regulator 5!. Between the motor 6! and shaft 62 we have found it to be desirable to utilize: a high ratio gearing 35 in order to reduce the speed ratio between the motor and the rotor 54. The motor 5| is provided with load brushes 6B and 61 and a field winding 58 which is separately energized from a constant source of voltage indicated by the plus and minus signs. With this arrangement the magnitude and polarity of the voltage impressed upon the armature brushes 56 and 6'! will determine the direction and speed of rotation of motor GI and consequently the direc- 6 tion and speed of rotation of rotor 54. Hence, a reversible polarity direct current generator 69 is utilized to energize the armature winding of motor 6!. Although various types of generators having the proper characteristics may be used, We have found it preferable to use a generator of the armature reaction excited type, known in the art as an amplidyne generator, and described and claimed in U. S. Letters Patent No. 2,227,992, granted June as, 1939, upon an application of M. A. Edwards and E. F. W. Alexanderson. The amplidyne generator is a preferred form because its low excitation requirements permit the use of relay and control switch contacts directly in its field circuits owing to the fact that the control field currents may be as low as 0.1 ampere for a generator terminal voltage of 300 volts and the resistance of the field circuits is low compared with that of conventional generator field circuits. The use of a reversible voltage genorator to energize the motor 5i permits the motor to be reversed without reversing contactors; permits easy adjustment of the motor armature voltage magnitudes; permits the use of simplified control circuits; and provides inherent dynamic braking. The generator 69 has a pair of short circuit brushes ill for providing the main armature reaction excitation of the machine and a pair of load brushes 1i and which are'in quadrature relation with the short circuit brushes ll). The generator is provided with a compensating winding "l3 in circuit with the load circuit brushes ii and E2 which substantially neutralizes the armature reaction of the generator along the axis of brushes ii and 12. The load brushes ii and are connected to the armature brushes and ill of motor 6i. The amplidyne generator is provided with three other field windings in addition to the compensating field winding 56. Two of these field windings W5 and '55 may be referred to as control field windings and the remaining field winding is is used as an antihunt and residual voltage killing field winding and is connected in shunt to the armature winding through an adjustable resistor H. The polarities of the field windings are such that with current flowing from left to right of the field winding, the right-hand load brush ii is assumed to be positive with respect to the left-hand brush l2. When brush 'H is positive, the motor 6i is assumed to. be rotated in the direction which causes the induction regulator rotor 5% to increase the power fiow from the circuit i to the circuit 6. For ease of identification, we have heretofore referred to circuit as a 60 cycle system and circuit 55 as a 25 cycle system. In this event, tube group it will be operating as a rectifier and tube group ll as an inverter. To obtain an increase in power flow from the 25 cycle systern to the cycle system, the rotation of the motor $5 is reversed by having current in the generator field. i l or it? flow from right to left. Control field is used only with a demand load regulator and control field M is used with either a manual control is or with a watt regulator" control 83, all to be described presently. The control fields and it have a common terminal 84 which is connected to a mid-voltage or neutral tap es of a suitable source of direct current such as a battery 83, although it will be obvious that conventional forms of midtapped rectifiers could be used without departing from our invention in its broader aspects. The other ends of the field windings 1 and '55 are connected through control contacts to be described presently to either end of the source 83 by means of an auxiliary control bus 84 so as to provide a simple means of obtaining current flow in either direction in the field windings.

Cam operated position switches 85, 86, 81 and 88 are operated from the shaft 62 of the motor GI in order to provide the desired control relative to the position of the rotor 54 of the induction regulator Position switch 85 includes a suitably shaped cam member 89, switch contacts 90, switch biasing means 9I and contact operating means 92 which may take the form of a rod positioned to engage the operating surface of cam 89. The surface of cam 89 is so designed that contacts 90 will be closed for motion of motor (H for substantially 165 mechanical degrees in a clockwise direction from a substantially twelve oclock position or Zero position corresponding to point A of Fig. 2 to, say, a five-thirty position corresponding to point X of Fig. 2. For the remainder of the circumference of the cam 89, contacts 90 are in the open position. During this clockwise movement of cam 89, power flow of the conversion system is assumed to be effected from the 60 cycle system (circuit I) to the 25 cycle system (circuit 6) Position switch 86 similarly includes a suitably shaped cam 93, switch contacts 94, switch biasing means 95 and a contact operating means 98. The surface of cam 93 is so designed that contacts 94 will be closed for motion of motor SI for substantially 165 degrees in a counterclockwise direction from a substantially twelve oclock position, or zero position, corresponding to point C of Fig. 2 to a six-thirty position corresponding to point Y of Fig. 2. For the remainder of the circumference of cam 93, the contacts 94 are in the open position. Position switch 81 is a limit switch and includes a cam 91, switch contacts 98, switch biasing means 99 and a contact operating means I00. The contacts 98 are arranged to be closed for a counterclockwise motion of motor BI beginning from a substantially five oclock position corresponding to point B of Fig. 2 to a fivethirty position approached from. the counterclockwise direction corresponding to point X of Fig. 2. In other words, contacts 98 are closed at all times except when the rotor 54 reaches the end of its travel in the 60 to 25 cycle power flow direction (clockwise) at which point the contacts 98 open. Position switch 88 is also a limit switch and includes a cam IOI, switch contacts I02, switch biasing means I03 and a contact operating means I04. 'Ilhe contacts I02 are arranged to be closed for a clockwise motion of motor 6| beginning from a substantially seven oclock position corresponding to point D of Fig. 2 to a sixthirty position approached from a clockwise direction and corresponding to point Y of Fig. 2. In other Words, contacts I02 are closed at all times except when rotor 54 reaches the end of its intended travel in the 25 to 60 cycle power flow direction (counterclockwise) at which point the contacts I02 open.

Indication of the position of the rotor 54 is obtained by mounting a Selsyn transmitter I05 on the shaft 62 and connecting the output of the transmitter to a Selsyn receiver I06 provided with position indicating means I01 and located at any convenient point for observation. The source of supply for the Selsyn indicator is indicated by conductors I08 of an auxiliary source of alternating current.

The contacts 98 and I02 of the two limit switches 81 and 08 are connected in series with unidirectional conducting devices, such as contact rectifiers I09 and H0, which are poled oppositely with respect to current flow from the midtap 02 of battery 83 so that in combination with their associated limit switches the field windings I4 and I5 are blocked against current flow which would run the rotor 54 past either limit of its travel.

An auxiliary relay 85a having an operating coil 85b and three sets of contacts III, H2 and H3 is associated with and controlled by position switch 85 so that the relay contacts are picked up for power flow from the 60 cycle system to the 25 cycle system, or from position A to position B in Fig. 2. Similarly, an auxiliary relay I I4 having an operating coil H5 and three sets of contacts I I8, I I1 and H8 is associated with and controlled by position switch 88 so that the relay contacts are picked up for power flow from the 25 cycle system to the 60 cycle system, or from position C to D in Fig. 2. Position switches 85 and 86 have their respective contacts 90 and 94 both open for a small portion of travel by the rotor 54 at the zero power flow position as indicated between A and C, Fig. 2. At the zero position of rotor 54, therefore, both auxiliary relays I09 and H4 are dropped out and all the contacts of the respective relays are in the open position. The use of the auxiliary relays 85a and H4 will be described later.

The circuit interrupting device I2 is provided with any suitable circuit closing means H9 and circuit openin means I20. The circuit opening means I20 may be made responsive to overcurrent conditions of the alternating current circuit of the conversion unit IS in a conventional manner but, in accordance with our invention, we provide an additional circuit I2I for energiza-tion in response to special fault conditions to be described later. The circuit interrupting device I2 is also provided with auxiliary circuit controllin contacts I22, I23 and I24 which are shown, for the open position of the circuit interrupting device I2, with contacts I22 and I24 open, and contacts I23 closed. In order to simplify the drawings, each of the contacts I22, I23 and I24 may be considered to represent two contacts in series, one contact for the 60 cycle end as shown and one corresponding contact for the circuit interrupter I5 of the 25 cycle end.

The rotor 54 of the induction regulator 5| is always in the position represented for zero power flow at the time the 60 cycle and 25 cycle circuit interrupts are closed. In the open position of circuit interrupters I2 and I5, as illustrated, which would also result if the circuit interrupters were tripped while the conversion apparatus was carrying load, the rotor 54 is automatically returned to the zero position corresponding to points A to C of Fig. 2 for zero power flow. This is accomplished by the auxiliary contacts I23 of circuit interrupter I2 which connects the control field winding I4, through contacts III or H6 of auxiliary relays 85a or 5 I4, to the polarity of battery 83 which causes the motor BI to drive rotor 54 towards its zero power flow position. As the rotor 54 arrives at a position corresponding to point A or C of Fig. 2, the relay 85a or H4 is deenergized and the rotor 54 remains in the zero power flow position.

Manual control of power flow through the conversion apparatus is efiected by means of the manual control apparatus I0 comprising a push button switch I25 with normally open contacts I26 for one direction of control, and a second push button switch I21 with normally open contacts I28 for the opposite direction of control. The switch I25 has its contacts I26 connected in series relation with an adjustable resistor I29 to the positive conductor of control bus 84, and switch I21 has its contacts I28 connected to the negative conductor of control bus 84 through an adjustable resistor I30. The magnitude of the current in the control field 14 and thus the speed at which the rotor 54 adjusts load setting is readily adjusted by the resistors I29 and I30. With circuit interrupters I2 and I5 in the closed position, interlocking contacts I22 are closed so that upon closure of manual control switch I25 the left-hand end of field winding 14 is connected to the positive side of control bus 84 while the right-hand terminal is connected through the contact 98 of auxiliary switch 81 and through rectifier I09 to the midtap of battery 82. This circuit effects energization of field winding 14 in a direction to cause rotor 54 to be operated in a clockwise direction from A to X, Fig. 2, and thus increase power flow from the 60 cycle system (circuit I) to the 25 cycle system (circuit 6). On the other hand, closure of manual control switch I21 connects the negative conductor of bus 84 through circuit interrupter interlock contacts I22 to the left-hand terminal of field winding 14 while the right-hand terminal thereof i connected through contact I02 of auxiliary position switch 88 and the rectifier IIO to the more positive midtap terminal 82 of battery 83. It will be observed that the polarity of field winding 14 has been reversed so that the motor 6| and hence rotor 54 would be operated in the reverse direction to cause an increase in power flow from the 25 cycle system to the 60 cycle system.

The first type of automatic control previously mentioned and identified in Whole as device 80 may be referred to as watt regulator control. This control is effected by means of an induction disc type device I3I comprising three watt responsive elements I32, I33 and I34. The elements I32 and I33 are connected in a manner similar to a conventional two-element threephase three-wire wattmeter. Th'us element I 32 is provided with a current coil I35 connected to be energized in accordance with the upper phase conductor of circuit II through a current transformer I36. This element is also provided with a voltage coil I31 connected to be energized in accordance Wtih the voltage of the upper two phase conductors of circuit, II through a potential transformer I38. The current winding I35 and voltage winding I31 are arranged in cooperative relation with an induction disc I39 to provide a torque proportional to the watts in this element. The element I33 is provided with a current coil I40 connected to be energized in accordance with the current in the lower phase conductor of circuit II through a current transformer I4I. This element is also provided with a voltage coil I42 connected to be energized in accordance with the voltage of the lower two phase conductors of circuit I I through a potential transformer I43. The current coil I40 and voltage coil I42 are arranged in cooperative relation with an induction disc I44 to provide a torque proportional to the watts in this element. The induction discs I39 and I 44 are mounted on a common shaft I45. In order to maintain the correct direction of torque on the watt regulator relay I3I for different directions of power flow, we have found it expedient to reverse the connections of the potential coils I31 and I42. As illustrated, auxiliary reversing relays I46 and Cal I41 are interposed between potential transformer I38 and its associated voltage coil I31 of element I32. Relay I46 is provided with an operating coil I48 which is connected to be energized in accordance with the operation of position relay 85a which, in turn, is controlled in accordance with the cam operated position switch 85 controlling power flow from circuit I to circuit 6. Relay I41 is provided with an operating coil I49 which is connected to be energized in accordance with the operation of position relay I I4 which, in turn, is controlled in accordance with the cam operated position switch 86 which controls power flow in the reverse direction, namely, from circuit 6 to circuit I. Similarly, auxiliary reversing relays I56 and I5I are interposed between potential transformer I43 and its associated voltage coil I42 of element I33. Relays I46 and I50 are arranged to be operated simultaneously by the operating coil I48, while relays I41 and I5I are arranged to be operated simultaneously by the operating coil I49. The two relay elements I 32 and I33 may be constructed in accordance with the principles of conventional and well known types of induction disc relays.

The third element of the watt regulator I3I is provided as a calibrating element and comprises a current winding I52 and a Voltage winding I53 which are arranged in cooperative relation with an induction disc I54 also mounted on the shaft I45. The current coil I52 is arranged to be energized with constant current and, as illustrated, is connected to be energized from a current transformer I55. The current transformer is connected in series relation with a resistor I56 across a secondary winding I51 of a constant potential transformer I58. An adjus able resistor or voltage divider I59 is also connected across the secondary winding I51 of the constant potential transformer and is provided with an adjusting arm I60. On terminal of the potential coil I53 is connected to the adjusting arm I60 and the other terminal is connected to the other side of the constant potential transformer. The voltage of the potential coil I53 is, therefore, variable between zero and full voltage of the transformer I58. By changing the setting of arm I60, it is possible to vary the torque producing watts in the calibrating element from zero to some predetermined value depending upon the constants of the circuit. The torque of calibrating element I34 act in the same direction as the torque of elements I32 and I33. An indicating wattmeter I6I, comprising a current cOil 5 I62 in circuit with current coil I52 and a voltage coil I63 responsive to the same voltage as voltage coil I53, indicates the watts input to the calibrating element I34 so that the indicating wattmeter may be calibrated to read directly the watt regulator setting.

The several contacts and cooperating auxiliary relays of the watt regulator apparatus may now be considered. The shaft I45 of the watt regulator device I3I per se is provided with a movable contact arm I64 which may be biased by suitable biasing means I65 to a position intermediate two pairs of cooperating contacts I66 and I61 and I68 and IE9. Contact I66 is connected to contacts H2 and I I1 of auxiliary relays a and H4, respectively, while contact I 68 is connected to contacts III and II 6 of auxiliary relays 85a and H4, respectively. Contacts I61 and I69 have a common junction I10 which is connected through a conductor IN to the lefthand terminal of field winding .14 of amplidyne generator 89. A switch I12 which may be referred to as a watt regulator transfer switch is connected in circuit with conductor I1 I. Switch I12, which may be automatically operated, is shown, for purposes of simplicity, as a manually operated switch and comprises one set of contacts I13 and a second set of contacts I14. The contacts I13 are open and the contacts I14 are closed when the switch is in one operating position such as the lower or off position as illustrated in the drawings. In the upper operating position or on position, as illustrated in the drawings, contacts I13 are closed and the contacts I14 are open. Contacts I13 are connected in series with con ductor HI and places this circuit under control of the watt regulator 80. Contacts I14 are connected in series with the contacts I26 and I28 of the previously described manual control switches I25 and I21, respectively. In all cases under watt regulator control, the desired direction of power flow must first be selected manually by the operator by means of switch I25 or I21. We may continue to trace the circuit of conductor I1I through closed contacts to be described presently through interlocking circuit interrupter contacts I22, which are closed when the power circuit interrupters I2 and I5 are closed, to the left-hand terminal of control field winding 14.

The regulator 18 may be referred to as a demand load regulator and for purposes of illustration may be assumed to be an indicating-integrating device designed to hold kilowatt hour demand practically constant at a predetermined value and at the same time hold the instantaneous kilowatt value within desired limits, Under the assumed system of connections generally outlined hereinbefore, it will be observed that there are three sources of power available for the industrial customers load 3, (a) the public utility 60 cycle system indicated by circuit I, (b) the industrial customers 60 cycle generators indicated by generator 5 and (c) the industrial customers cycle generators indicated by generator 1 feeding into bus 6. The electronic conversion apparatus 9 interconnects the 25 cycle system, circuit 6, and the 60 cycle system, circuit I. The demand load regulator 18 is connected to respond to the net interchange of power between the public utilitys 60 cycle system, circuit I, and the power system of the industrial customer. The demand load regulator may be of a type commercially available in the art and it is believed sufficient for the purpose of disclosing our invention to indicate the regulating instrument per so by the rectangular symbol I15. The current element of the regulator I15 is energized through a switch I15 from current transformers I16 connected at the tie point I11 between circuit I and the connection point of the industrial customers circuit 4 and electronic converter bus II. The voltage element of the device I15 may be energized through a suitable potential transformer I18 connected to respond to the voltage at the tie point I11 or, as illustrated, may be connected to the bus 4 which has a voltage of predetermined phase, frequency and magnitude relative to the voltage of the tie point I11. The device I15 is provided with a movable contact arm I19 which is biased by suitable means I80 to a position intermediate two groups of contacts I8I and I82 constituting one group and I83 and I84 constituting the second group. The outer limit contacts I8I and I84 may be identified as the fast, raise and lower contacts, respec- 12 tively, while contacts I82 and I83 may be identifier as the slow, raise and lower contacts, respectively. The raise contacts I8I and I82 are connected through adjustable resistors I85 and I86, respectively, to the positive side of control bus 84. The lower contacts I83 and I84 are connected through adjustable resistors I81 and I88, respectively, to the negative side of control bus 84. Separate resistors for raise and lower speed adjustment are used since operating engineers may find it desirable to decrease the load setting at a faster rate than the load setting is increased, or Vice versa.

To connect the demand load regulator into the control circuits heretofore described and at the same time switch out other circuits which might cause conflicting control, we provide a demand load regulator transfer switch I89. This switch is a two position switch and is provided with three sets of contacts I98, I9I and I92. In the lower or off position of switch I89, when the demand load regulator 18 is not intended to effect a controlling action, the contacts I and I9I are in a closed position. Contacts I98 are in series relation with conductor I1I, making it possible for watt regulator 80 to exercise control, and contacts I9I make it possible for manual control to be exercised through manual control switch 19. When switch I89 is in its upper or on position, the contacts I90 and I9I are in the open position disabling both manual control 19 and watt control 89 and putting demand load regulator 18 in control. As previously pointed out, the demand load regulator 18 exercises its control through field winding 15 rather than field winding 14 which is utilized with man ual control 19 and watt regulator 80. Thus, with contact I92 closed, a circuit can be completed from either the positive or negative conductor of control bus 84, depending upon the position of the regulator arm I19, through interlocking contacts I24 of circuit interrupters I2 and I5 through a conductor I93 to the left-hand terminal of winding 15. The right-hand terminal of winding 15 is connected through junction terminal 8|, which in turn is connected through the contacts of limit switches 81 or 88 to the midtap 82 of battery 83.

The usual types of faults to which electronic rectifier apparatus is subjected, such as arebacks, may be taken care of successfully by ignitor blocking in, accordance with the system described and claimed in U. S. Letters Patent No. 2,348,653, granted May 9, 1944 upon an application of A. H. Mittag and assigned to the assignee of the present application. In the ignitor blocking system, means are provided to short circuit the ignitor firing impulses to an entire tube group. We have indicated such blocking for the ignitors of tube groups I6 and 11 by the ignitor blocking relays 43 and 43', respectively, heretofore described. The relays 43 and 43 are controlled by a fault responsive relay I94. The relay I94 is provided with an operating coil I95, three sets of normally open contacts I96, I91 and I98, and a normally closed set of contacts I99. The contacts I96 and I98 constitute the ignitor blocking control contacts and when closed cause energization relays 43 and 43', respectively, over control circuits 200 and 20 I, respectively, to short circuit the impulses to the ignitor. The contact I99 is connected in series with the conductor to the midtap 82 of control battery 83 so that upon closure of contacts I and I91 for ignitor blocking the control of the phase shifting apparatus will also be interrupted through the interruption of the control field windings of generator 69. For arc-back protection of each rectifier, it is satisfactory to use an instantaneous overcurrent relay responsive to the alternating current supply to the respective converters I6 and I? operating as rectifiers. We may obtain this response from the current transformers 2% or 293 through selective switch 204 which permits connection of the fault responsive apparatus to whichever end of the converter is operating as a rectifier. Although various known types of overcurrent relays may be used, we have shown an overcurrent relay 285 provided with an operating coil 2% connected to be energized from a resistor 20'! which, in turn, is energized through a full wave rectifier 298 having its input side connected by selective switch @26 to either transformer 292 or 203. Relay 205 is provided with normally open contacts 269 which when closed complete a control bus circuit indicated by the plus and minus signs to cause energization of operating coil I95 of the fault responsive relay 494.

Since an arc-back is also manifested in the direct current circuit of the electronic converter, we provide a current shunt 2 I in the direct current loop- 3t and through conductors 2i I energize a second relay 252 having normally open contacts Elli. The contacts 2&3 are connected in parallel relation with contacts 289 so that operation of either relay will cause energization of the ignitor blocking relays it and 43. Relay 2 l2 also affords protection for a fault condition of inverter operation which is reflected on the direct current circuit and which may not be reflected in the alternating current circuits of the conversion unit. Under certain fault conditions of the inverter, such as tube failures, misfire, or insufficient deionization time, two tubes connected back-to-back, such as the right-hand pair of tubes !8 in group I5, may be simultaneously conductive during inverter operation. This condition is known as a shoot-through and constitutes a short circuit on the direct current loop but may not result in abnormally high currents in the inverter transformer. In this case, the ignitor blocking relays t3 and 43 would be picked up by the operation of relay ZIZ. Thus either an arc-back or an inverter shoot-through will be successfully cleared by the ignitor blocking relay.

In the case of repeated arc-backs or shootthroughs, the fault suppression apparatus will merely suppress these faults as fast as theyoccur. However, it has been found desirable to disconnect the converter when the fault responsive means operates a predetermined number of times within a predetermined period of time. In accordance with our invention, We provide means for performin this function. For this purpose, we have illustrated a known form of notching relay ZM which is provided with an operating coil 2l5, a ratchet wheel 2I6, a reciprocating lever mechanism 2 l l, a pawl 2 I8, a delayed time release 2I9 for the pawl, and a contact arm 220 operated by the ratchet wheel. The contact arm is biased t predetermined starting position against stop 22I by spring 222. The operating coil H5 is connected to be energized from a control bus 223 through contact I91 of the ignitor blocking control relay I94 each time the ignitor blocking control relay is operated to its closed or upper position as illustrated in Fig. la of the drawings. When coil 2I5 of the notching relay is energized, it moves the mechanism 2H 50 as to notch the ratchet wheel Zlfi one notch in a counterclockwise direction and at the same time operates the time mechanism 2l-9 to release the pawl 2I8 and thereby hold the ratchet wheel in the new position. If the number of impulses or energizations of coil 2I5 are sufficient to move contact arm 2% to a predetermined position before the predetermined time has elapsed, a pair of contacts 224 in series relation with conductor lZI are closed so that tripping mechanism I20 of circuit interrupter l2 will be energized and the circuit interrupter will be operated to its open position. Otherwise, if the frequency of occurrence of ignitor blocking is less than the predetermined time, the timing mechanism will reset pawl 2I8 to a releasing position and the contact arm will return to its biased position against stop ZZI without effecting operation of the releasing mechanism E20 of circuit interrupter i2.

The Operation of the embodiment of our invention illustrated in Figs. 1a and 1b is substantially as follows: The electronic conversion unit 9 is switched into service by closing the power circuit interrupters i2 and I5, thereby connecting the transformers It and E3 of the electronic converter unit to the systems I and 6, respectively. The switches it and 416 are closed to energize the excitation control units 36 and 36, and switch 3' is closed. to interconnect the 60 cycle system i of the industrial customer with the G0 cycle system of the public utility system at the tie point Ill. Switch I75 is closed to energize the potential coil of demand load regulator H5. The only prerequisite to putting the unit into service is that the excitation control units 36 and 36 for both ends of the converter be established for inverter operation. Since the operation of the various control and regulating devices has been described hereinbefore relative to their individual functions, it is believed an overall description of the operation of the system under control of the several control means will sufiice for a clear understanding of the illustrated embodiment.

When the switching of the power circuits and auxiliary power circuits has been effected as outlined above, the power converter is ready for control of direction and magnitude of power flow. The operator has a choice of three types of control: manual control by device l9, watt regulator control by device and demand load regulator control by device 78. The direction of power flow through the electronic converter 9 is determined by the connections of the phase shifting networks 36 and 36'. These connections are made by the R and I positions of switches 59 and 50. When the switch 50 is moved to the I position, the phase shifting network 36 causes the tube group E6 to operate as an inverter, whereas when the switch 58 is moved to the R position the phase shifting network 36 causes the tube group I6 to operate as a rectifier. A corresponding operation is obtained with respect to switch 50 and its associated phase shifting network 33 and the tube group ll. Therefore, for example, to have power flow through the converter 9 from the 60 cycle system to the 25 cycle system, switch 50 would be moved to the R position and switch 58' would be moved to the I position. The magnitude of the power flow through the electronic converter 9 is determined by the magnitude of the reference voltage derived from induction regulator 5| by means of the resistors 48 and 48 associated with the phase shift networks 36 and 36'. This reference voltage is determined by the position of the rotor 54 of induction regulator 5| so that control of the rotation of the rotor 54 is the direct means by which power flow through the electronic converter '9 is controlled.

Manual control of power flow is by means of the simple double throw momentary contact control switch 19. Field winding 14 of amplidyne generator 69 is used for manual control. Thus if switch I is closed, the left-hand terminal of field winding 14 is connected to the positive terminal of control bus 84 through contacts I14, contacts I9I, auxiliary contacts I22 of circuit interrupter I2, and through conductor I1I. This direction of energization has been assumed to cause clockwise rotation of motor BI and thereby effect an increase in power flow through the electronic converter from circuit I to circuit 6. The closure or switch I21 causes the reverse operation by connecting the left-hand terminal of field winding 14 to the negative terminal of bus 84. An operator, therefore, merely selects the desired position of control switch 19 to give the desired direction of power flow and the switch is held in that position until power flow has increased to the desired magnitude, such as may be indicated by any conventional wattmeter on the control board. It will be observed, however, that with current flow from left to right, in field winding 14 the path from the right-hand end or junction 8| to the battery neutral terminal 82 is through contact 98 of position switch 81 and rectifier I09. Thus when rotation of cam 91 has been effected in a clockwise direction to a point corresponding to point B in Fig. 2, the field circuit is interrupted by switch 98 and no further increase in power flow can be effected beyond the established limit by this means. A similar limit is established at point D, Fig. 2, for power flow in the reverse direction by means of switch 88 and its cam IUI which opens contact I02 in the circuit from neutral battery terminal 82 through rectifier I Ill.

The watt regulator 89 is a means for automatically regulating the power flow through the electronic converter 9. The power elements I32 and I33 are connected in the power circuit of the electronic converter in the manner described above with the potential coil reversing switches I48 and I49 arranged to connect the potential elements automatically and correctly in accordance with the operation of position switches 85 and 86. The regulator 80 has two sets of contacts; one set I68, I69, which has been identified as the lower set, is closed when the electronic converter power flow is greater than the regulator calibration, and another set of contacts I66, I61, which has been identified as the raise set, is closed when the electronic converter power is less than the regulator calibration. Both sets of contacts are open when the electronic converter power flow is equal to or within a predetermined percentage of the regulator calibration. Calibration is changed by the adjusting arm I69 of the calibrating resistor I59. The indicating instrument I6I may be arranged to indicate the calibration setting directly in kilowatts. In putting the electronic converter on watt regulator control, it is initially necessary for the operator to select direction of power flow by means of the manual switch 19. Once the rotor 54 of induction regulator 5| arrives at a position corresponding to position A or C in Fig. 2, the watt regulator transfer switch I12 is moved to its upper or on position closing contacts I13 and the watt regulator functions to pick up the load for which the regulator is set. This operation of the transfer switch I12 disconnects the manual control through the opening of contacts I14. It will be assumed that the operator has moved rotor 54 to a position corresponding to position A so that position switch 85 has been moved to close cntacts 90 and power flow has been initiated through the electronic converter from circuit I to circuit 6. With the closure of contacts 98, auxiliary switch 95a is picked up and closes its contacts I I I, II2 and I I3. Contacts III connect the lower contact I68 to the negative conductor of control bus 84 and contacts II2 connect the raise contact I66 to the positive conductor of the control bus 84. With the power flow established through the converter from cirj cuit I to circuit 6, it will be assumed that the power flowing is less than the setting of the "Watt regulator. Its contact arm I64 would be moved to close raise contacts I66, I61, which results through contact I I2, in connecting the left-hand 1 terminal of field winding 14 to the positive terminal of control bus 84 and thus causes an increase in the power flow through the electronic converter from circuit I to circuit 6. The operation which follows when the power fiow through the converter is greater than the watt regulator setting will be obvious as the lever I64 moves to close the lower contacts I68, I69 to reverse the polarity of field winding 14 and thereby reverse the direction of rotation of rotor 54 of induction regulator 5I. A sequence of events similar to that just described will occur if the power flow is in the reverse direction through the converter. In this case contact 94 of position switch 86 will be closed resulting in picking up relay I I4. This action reverses the connection of the potential coils of watt regulator 89 for the reverse power flow and at the same time reverses the connection of field winding 14 so that an increase in power flow is effected by a counterclockwise rotation of motor 6| and a decrease of power flow is efiected by a clockwise rotation.

The demand load regulator is put into operation by moving the demand regulator transfer switch I89 to its on or upper position as illustrated in the drawings. At the same time, the watt regulator transfer switch I12 is moved to its oif or lower position as illustrated in the drawings. With switch I89 in its on position, its contacts I90 and I9I are opened cutting out the watt regulator control and at the same time contacts I92 are closed connecting the demand regulator contacts I8I to I84 in condition for effecting control. The demand load regulator I8 permits the industrial customer to make the best possible exchange of kilowatt hours with the public utility system as determined by the customer's contract with the power company. When the 60 cycle industrial load 3 is greater than the setting of the regulator I8, the elec tronic power converter 9 is controlled to supply power from the customers 25 cycle system, circuit 6, to the 60 cycle system. On the other hand, when the 60 cycle industrial load 3 is less than the setting of the regulator 18, the electronic power converter 9 is controlled to supply power from the 60 cycle system to the customers 25 cycle system. This control is effected through field winding 15, rather than field winding 14, of the amplidyne generator 6I. Assume that when the system is put under demand load regulator control the integrated load demand of the 60 cycle industrial load is below the setting of the regulator 18. These conditions require that the power flow to the industrial customer must increase and this is eifected by causing the electronic converter to transmit power from the 60. cycle system to the 25 cycle system. Therefore, the movable arm I19 of the regulator 73 moves to close one of the raise contacts I32 or IE! and thereby connects the left-hand terminal of field winding il to the positive conductor of control bus 8 5. This connection results in a clockwise rotation of motor and results in power flow from the 60 cycle system to the 25 cycle system in the direction desired. Conversely, if the integrated load is above the setting of the regulater '18 the contact arm I19 is moved to close one of the lower contacts ass or iii! and thereby reverses the connections of field winding it. This connection results in a counterclockwise rtation of motor 8| and results in power flow from the cycle system to the 60 cycle system in the direction necessary to bring the integrated demand down to the setting of the "demand regulator.

In Figs. 3a and I), considered together, we haveshown an embodiment of our invention arranged for controlling a system in which a plurality of electronic converters are utilized to interconnect the assumed 60 cycle and 25 cycle systems. The demand load regulator ill of the single con verter system is arranged to control both elec= tronic converters, while means are provided to keep the loads on the two electronic converters approximately equal or in a predetermined ratio under these conditions of control. Ihe arrangement also illustrates the system with watt regu lator control for each electronic converter unit and with means for adjusting the calibration of each watt regulator. Although the control circuits of each converter unit would be provide with the manual control 19, as illustrated in Figs. la b, it has been omitted from the illustration of this embodiment in order to simplify the drawings. The various devices and elements of Figs. 3a and 32) corresponding to those of Figs. la and 1b and the electronic converter there she Jl'l have been assigned like reference numerals, whereas corresponding devices and elements of the additional electronic converters have been assigned like reference numerals which have been primed and in a few instances double primed where the devices were originally desi nated by a primed numeral.

The system as illustrat d therefore, comprises the assumed 60 cycle circuit I of the public utility, the Si) cycle system l of the industrial customer, both of which are interconnected with the 25 cycle system of the industrial customer which is indicated b circuit 5. In the arrangement as illustrated two electronic converter circuits, indicated by the buses H and ii and including the electronic converters 9 and 9', interconnect the 69 cycle system and the 25 cycle system. Each of the electronic converters 9 and 9' is provided with control apparatus corresponding to the apparatus illu"*-'ated in Figs. 1a and b and except for the addi cnal elements and devices which have been introduced in this modification, it is not believed that further description is required for an understanding of the invention.

In connection with the watt regulators ill and 8% means are provided to adjust the calihration of these regulators simultaneously and this may be accomplished through the calibrating rheostats E59 and IE9 by means of a shaft 225 having the contact arms E66 and Hill mounted thereon. A reversible driving means, such as the reversible motor 226, is arranged to operate the shaft 225. The motor may be any suitable form of reversible motor and for purposes of il lustration is shown as a direct current motor with a split field winding comprising sections 221 and 223 connected to be selectively energized from a source of direct current 229 through a selector switch 230. The switch 23!] may be located at any convenient point for purposes of remote control and in practice has been mounted on a control panel so that the operator can View the indicating wattmeters lol and I55 and thereby determine by observation the desired calibration setting.

Each of the calibrating elements of the watt regulators to and 88' may be provided with a load balancing adjuster control which in the case of watt regulator 89 comprises an adjustable resistor 23! connected across the voltage coil I53 and similarly in watt regulator adjustable resistor 23! connected across the voltage coil I53. This control is used only when the electronic converters are both under watt regulator control. Any unbalance of power on the two electronic converters can be corrected by properly adjusting resistors till end 23l. The load balance adjusters and 231' should normally be in their zero resistance positions and if it is necessary to balance the load between the two electronic converters, the resistor for the unit carrying the smallest load need be the only resistor changed from the zero position.

When the electronic converters Q and 9 are under control of the demand load regulator it, it may be desirable to provide means to maintain automatically a predetermined division of load between the respective converters. For this purpose, we provide load balance relays 232 and 232,

and for purposes of explanation it will be as-.

sumed that it is desired to maintain equal loads on the respective converters. The load balance relay 232 may be a conventional polyphase induction disc type relay with current coils 233 and 234 and Voltage coils 235 and 236. The current and voltage coils 233 and 235 are arranged in cooperative relation with an induction disc 23? and the current and voltage coils 231i and 235 are similarly arranged in cooperative relation with an induction disc 233. The discs 23'! and 238 are mounted on a common shaft 239. Load balance relay 232" is constructed in a similar manner with current and voltage coils 233' and 235' in cooperative relation with disc 23'? and current and voltage coils 234 and 236' in cooperative relation with disc 238 with the discs mounted on a common shaft 23%. Current transformers 2 18 and 246' are connected, respectively, in the corresponding outside phase conductors of circuits H and H and are arranged in differential relation to energize current coils 233 and 233', Similarly current transformer 2M and 24! are connected in the corresponding inside phase conductors of circuits ii and ii and are arranged in differential relation to energize current coils 23d and 23B. Voltage coil 235 is connected through a potential transformer M2 to the phase voltage of its associated current coil 233 of circuit H and voltage coil 23% is connected through potential transformer 253 to the phase voltage of its associated core coi 23 i. Voltage coils 235' and 235' are similarly connected to circuit l I through tential transformers 252' and E l-3'. The shaft 239 of relay 232 is provided with a movable contact arm M l biased by suitable means 245 to a position intermediate one pair of "raise contacts 245 and 241 and a second pair of lower contacts 243 and 24?. The shaft 239' of relay 232' is similarly constructed with a movable contact arm 244'. bias means 245 and cooperating raise contacts 248-24? and lower contacts Mir-249C The interconnected or common contacts 246 and 248 of relay 882 are connected through a pair of con tacts 258 which have been added to watt reg ulator transfer switch I12. By reason of the addition of the load balancing relays 232 and 232, additional contacts are required on the demand load transfer switches I89 and I89. These contacts are identified as 25! and 252 on switch I89 and. 25! and 252' on switch I89. Thus from contacts 258 of watt transfer relay I12, the control circuit may be traced through contacts 252 of relay I89, through contacts 25! of relay 188, through contacts I22 of circuit interrupter l2 to the right-hand terminal of field winding 14 of amplidyne generator 69. Proceeding in the opposite direction from contacts 250 this right-hand terminal of winding 14 is then connected to the positive conductor of bus 84 through auxiliary relay 85a or the negative conductor of bus 84 through auxiliary relay I 14, depending upon the operation of position switches 85 and 86 and whether the load balancing relay 232 is in its raise or lower position. The opposite or left-hand terminal of field winding 14 is conected through the limit switches 81 or 88 to the neutral terminal 82 of the battery 83. The left-hand terminal of field winding 14' of amplidyne generator 69' is similarly connected to the plus or minus conductor of bus 84, depending upon the operation of the position switches 85 and 86 and whether the load balance relay 232 is in its raise or lower position. right-hand terminal of field winding 14 is connected through limit switches 81' or 88' to the neutral terminal 82 of battery 83. With equal loads on the respective electronic converter 9 and 9' and with the differential arrangement of the current transformers of the balance relays 232 and 232', no current flows in the current coils of either relay and the relays fioat.

In analyzing the operation of relays 232 and 232, it is necessary to keep in mind that the motor 6|, amplidyne generator 69 and the position switches in Fig. 3a are reversed in position. as illustrated, from that of the corresponding elements in Fig. 3b and also from th cor responding elements in Fig. 1b. Hence, it is necessary in Fig. 3a to reverse the convention in regard to efiecting a decrease or increase in power flow between the respective alternating current circuits. The convention may be correctly kept in mind by considering a mirror image of Fig. 2 so that when power fiow is from the 60 to 25 cycle system a counterclockwise rotation of posi tion switch 85 effects an increase in power flow and a clockwise rotation a decrease. Converse- 1y, when power fiow is from the 25 to 60 cycle system, a clockwise rotation of position switch 85 effects an increase in power fiow and a counterclockwise rotation a decrease. tion with respect to the direction of rotation of motor 6| in relation to the direction of energize." tion of field winding 14 remains the same since current flow from right to left is assumed to cause a counterclockwise rotation of motor 6! and energization from left to right is assumed to cause a clockwise rotation. In connection with motor BI and amplidyne generator field 14 of Fig. 3b the same convention is observed as for the corresponding elements in Fig. 1b.

With the stated conventions in mind the operation of the load balance relays 232 and 232' The The conven- Cir will be briefly tabulated for unbalance of load on the respective electronic converters 9 and 9. The relays are connected to close their contacts as follows:

Power flow 60 cycle system to 25 cycle system Converter 9 carrying greater load 246'-241 closed Power flow 25 cycle system to 60 cycle system Converter 9 carrying greater load Relay 232 contacts 25.6-248 closed,

246-241 open Relay 232' contacts 246-241 closed, contacts 246'-248' open Converter 9 carrying greater load Relay 232 contacts 248-241 closed,

246-248 open Relay 232' contacts 24B'-248 closed, contacts 246-24'|' open A study of Figs. 3a and 3b and the above tabulation will show that in all cases the relays 232 and 232 operate to balance the power flow for the two electronic converters. For example, assume that power flow is from the 60 cycle to the 25 cycle system and that converter 9 is carrying the greater load. This requires that relay 232 reduce the load of converter 8 and increase the load of converter 9. For the assumed direction of power fiow and reversed convention for the control apparatus of Fig. 3a, motor 8| will be required to rotate in a clockwise direction which requires that field winding 14 be made negative at the right-hand terminal. When It, lay 232 moves to close its lower contacts 246-248, relay I89 is picked up and connects the right-hand terminal of field winding 3 to the negative conductor of bus 84 thereby reducing the power flow of converter 9. At the same time, relay 232 moves to close its raise contacts 246-241' which effects a clockwise rotation of motor 6| and thereby raises the ower fiow through converter 9. When the load on the two electronic converters becomes balanced, the relays 232 and 232 open their contacts and the contact arms fioat in the neutral position. The

contacts contacts demand load regulator 18 functions to control both converters in the manner described hereinbefore so as to maintain the net interchange of power at the desired value determined by the demand load setting.

While we have shown and described particular embodiments of our invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from our invention and we, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In an electronic conversion system, a pair of circuits one of which is an alternating current circuit, means comprising electric translating apparatus interconnecting said circuits and comprising a plurality of electronic tubes each having' an anode, a cathode and a control electrode, means having a rotatable element for controlling the potential of said control electrode to determine the instant of conduction between the anode and cathode of each tube, a direct current motor connected to operate said movable element, a direct current generator of the armature reaction-excited type connected to energize said motor and having a pair of short circuit brushes and a pair of load circuit brushes displaced therefrom, a control field winding for said generator, and means connected to said control field winding and responsive to an electrical condition of said conversion unit for controlling the power interchanged between said pair of circuits.

2. In an electronic conversion system, a pair of circuits one of which is an alternating current circuit, means comprising electric translating apparatus interconnecting said circuits and comprising a plurality of electronic tubes each having an anode, a cathode and a control electrode, means having a rotatable element for controlling the potential of said control electrode to determine the instant of conduction between the anode and cathode of each tube, a direct current motor connected to operate said movable element, a direct current generator of the armature reaction excited type connected to energize said motor and having a pair of short circuit brushes and a pair of load circuit brushes displaced therefrom, a control field winding for said generator, and means for selectively varying the polarity of the voltage of said generator for operating said motor in either direction of' rotation comprising means for reversibly energizing said control field winding.

3. In combination, a pair of electric circuits, means interconnecting said circuits and comprising a movable element operable from a first predetermined position to a second predetermined position in one direction of rotation for effecting a change in one sense between an electrical condition of said circuit and operable from said first predetermined position to a third predetermined position in the opposite direction of rotation for efiecting a change in the opposite sense between said electrical condition of said circuits, a direct current motor reversible in its direction of rotation for operating said movable element, a reversible polarity direct current generator provided with a control field winding and connected to energize said motor, a source of direct current voltage for energizing said control field winding, means for reversing the polarity of the energization of said control field winding from said source, and means for selectively determining the extent and direction of rotation. of said motor in accordance with the position of said movable element,

4. In combination, a pair of electric circuits, phase shifting means interconnecting said circuits and comprising a movable element operable from a first predetermined position to a second predetermined position in one direction of rotation for effecting a change in. phase in one sense between the voltage of said circuits and operable from said first predetermined position to a third predetermined position in the opposite direction of rotation for effecting a change in phase in the opposite sense between the voltages of said circuits, a direct current motor reversible in its direction of rotation for operating said movable element, a reversible polarity direct current generator provided with a control field winding and connected to energize said motor, a source of dimet current voltage for energizing said control field winding, means for reversing the polarity of the energization of said control field winding from said source, and means operated by said motor for selectively determining the extent and direction of rotation of said motor in accordance with the position of said movable element.

5. In a control system, a dynamo-electric machine, a control field winding therefor having two terminals, means for energizing said field winding comprising a source of unidirectional voltage having positive and negativ terminals and an intermediate terminal, means comprising a twobranch circuit for interconnecting one terminal of said field winding to said intermediate terminal and having in one branch a circuit interrupting device and a unidirectional conducting device poled in a direction for current flow through said field winding in one direction and having in the other branch a circuit interrupting device and a unidirectional conducting device poled in a direction for current flow through said field winding in the opposite direction, and means for selectively connecting the other terminal of said field winding to the positive or negative terminal of said unidirectional source.

6. In a control system, a direct current generator having a control field winding provided with two terminals, means for energizing said field winding comprising a source of unidirectional voltage having positive and negative terminals and an intermediate terminal, means comprising a two-branch circuit for interconnecting one terminal of said field winding to said intermediate terminal and having in one branch a circuit interrupting device and a unidirectional conducting device poled in a direction for current flow through said field winding in. one direction and having in the other branch a circuit interrupting device and a unidirectional conducting device poled in a direction for current flow through said field winding in the opposite direction, means for selectively connecting the other terminal of said field winding to the positive or negative terminal of said unidirectional source, and means for selectively controlling said circuit interrupting devices.

7. In a control system, a reversible polarity direct current generator having a separatel energized control field winding, a direct current motor connected to be energized from said generator and having a direction of rotation dependent upon the polarity of said generator, means for energizing said field winding comprising a source of unidirectional voltage having positive and Mg ative terminals and an intermediate terminal, means comprising a two-branch circuit or interconnecting one terminal of said field winding to said intermediate terminal and having in one branch. a circuit interrupting device and a unidirectional conducting device poled in a direction for current flow through said field winding in one direction and having in the other branch a circuit interrupting device and a unidirectional conduct ing device poled in a direction for current fiow t rough said field winding in the opposite direction, for selectively connecting the other terminal of said field winding to the positive or tive terminal of said unidirectional source, means operative in response to the rotation of said motor in one direction for controlling said circuit interrupting device of said one branch, and means operative in response to the rotation of said motor in the other direction for control.

23 ling said circuit interrupting device of said other branch.

8. In a control system, a reversible polarity direct current generator having a separately energized control field winding, a direct current motor connected to be energized from said generator and having a direction of rotation dependent upon the polarity of said generator, means for energizing said field winding comprising a source of unidirectional voltage having positive and negative terminals and an intermediate terminal, means comprising a two-branch circuit for interconnecting one terminal of said field winding to said intermediate terminal and hav ing in one branch a circuit interrupting device and a unidirectional conducting device poled in a direction for current flow through said field winding in one direction and having in the other branch a circuit interrupting device and a unidirectional conducting device poled in a direction for current flow through said field winding in the opposite direction, means comprising a pair of control circuits each having two circuits arranged for selectively connecting the other ter-- minal of said field winding to the positive or negative terminal of said unidirectional source, means operative in response to the rotation of said motor in one direction for controlling said circuit interrupting device of said one branch during control of said motor in one direction for either corresponding circuit of said pair or" control. circuits, and means operative in response to the rotation of said motor in the other direction for controlling said circuit interrupting device of said other branch during control of said motor in te other direction for either of the other circuits of said pair of control circuits.

9. In a control system, a reversible polarity direct current generator having a separately energized control field winding, a direct current motor connected to be energized from said genorator and having a direction of rotation dependen upon the polarity of said generator, means for energizing said field winding comprising a source of unidirectional voltage having positive and negative terminals and an intermediate terminal, means comprising a two-branch circuit for interconnecting one terminal of said field winding to said intermediate terminal and having one branch a circuit interrupting device and midirectional conducting device poled in a direction for current fiow through said field windir in one direction and having in the other branch a circuit interrupting device and a unidirectional conducting device poled in a direction for current fiow through said field winding in the opposite direction, means comprising a pair oi manual control circuits and a pair of automatically controlled circuits each having two circuits arranged for selectively connecting the other term 1a]. of said field winding to the positive or negative terminal of said unidirectional source, means operative in response to the rotation of said, motor in one direction for control ling the circuit interrupting device associated" with the unidirectional conducting device transmittng current in one direction under either said manual or automatically controlled circuits, and means operative in response to the rotation of said motor in the other direction for controlling the circuit interrupting device associated with the unidirectional conducting device transmitting current in the other direction under either said manual or automatically controlled circuits.

10. In a control system, a direct current dyna-' moelectric machine having a plurality of control field windings each provided with two terminals, means for energizing said control field windings comprising a source of unidirectional voltage having positive and negative terminals and a terminal intermediate thereof, means compris ing a two-branch circuit for interconnecting one terminal of all of said field windings to said intermediate terminal and having in one branch a unidirectional conducting device poled in a direction for current flow from said field, windings to said intermediate terminal and having in the other branch a unidirectional conducting device poled in a direction for current flow from said intermediate terminal to said field windings, and means for reversing the current flow in the respective field windings comprising means connected to the other terminal of each field winding for selectively connecting said other terminals of its associated field winding to the positive or negative terminal of said source of unidirectional voltage.

11. In a control system, a direct current dynamoelectric machine having a plurality of control field windings each provided with two terminals, means for energizing said control field winding comprising a source of unidirectional voltage having positive and negative ter ninals and a terminal intermediate thereof, means comprising a two-branch circuit for interconnecting one terminal of all of said field windings to said intermediate terminal and having in one branch a unidirectional conducting device poled in a direction for current fiow from said field windings to said intermediate terminal and having in the other branch a unidirectional conducting device poled in a direction for current flow from said intermediate terminal to said field windings, means for reversing the current flow in the respective field windings comprising a pair of control circuits connected to the other terminal of each field winding for selectively connecting said other terminals of its associated field winding to the positive or negative terminal of said source of unidirectional voltage, and

adjustable resistance means connected in each circuit of at least one of said pair of control circuits.

12. In a control system, a direct current generator of reversible polarity having a plurality of control field windings each provided with two terminals, means for energizing said field windings comprising a source of unidirectional voltage having positive and negative terminals and an intermediate terminal, means comprising a two-branch circuit for interconnecting one terminal of each of said field windings to said intermediate terminal and having in one branch a unidirectional conducting device poled in a direction for current flow through each field winding in one direction and having in the other branch a unidirectional conducting device poled in a direction for current flow through said field windings in the opposite direction, and means for selectively connecting the other terminal of each field winding to the positive or negative terminal of said unidirectional source for reversing the polarity of said generator.

13. In a control system, a direct current generator of reversible polarity having a plurality of control field windings each provided with two terminals, means for energizing said field windings comprising a source of unidirectional voltage having positive and negative terminals and an intermediate terminal, means comprising a twobranch circuit interconnecting one terminal of each of said field windings to said intermediate terminal and having in one branch a unidirectional conducting device poled in a direction for current fiow through each field winding in one direction and having in the other branch a unidirectional conducting device poled in a direction for current flow through said field windings in the opposite direction, means comprising a pair of control circuits for selectively connecting the other terminal of each field winding to the positive or negative terminal of said unidirectional source for reversing the polarity of said generator, and adjustable resistance means connected in each circuit of said pair of control circuits,

14. In combination, a reversible polarity direct current generator having a separately energized control field winding, a direct current motor connected to be energized from said generator and being operable in opposite directions of rotation in dependence upon the polarity of said generator, a source of voltage for energizing said field Winding, means for selectively energizing said field winding from said source for reversing the polarity of said generator, and adjustable means in circuit with said last mentioned means for causing operation of said motor at different speeds for opposite directions of rotation.

15. In combination, a rotatable element for controlling an electrical condition of an electric circuit, a reversible electric motor for operating said rotatable element in oppositedirections from a predetermined neutral position for efiecting the controlling action of said rotatable element, a generator provided with a control field winding and having one polarity or the reverse thereof in dependence upon the direction of energization of said control field winding, said generator being connected to energize said motor, means for reversibly energizing said control field winding, and means responsive to interruption of current flow in said electric circuit for causing energization of said field winding in a direction to return said rotatable element to said neutral position.

16. In combination, an electric circuit, circuit interrupting means connected in said electric circuit, means for controlling an electrical condition of said electric circuit comprising a, rotatable element, a reversible electric motor for operating said rotatable element in opposite directions from a predetermined neutral position for effecting the controlling action of said rotatable element, a generator provided with a control field winding and having one polarity or the reverse thereof in dependence upon the direction of e-nergization of said control field winding, said generator being connected to energize said motor, means for selectively energizing said control field winding in one direction or the other, and means operative in response to operation of said circuit interrupting means to interrupt said electric circuit for causing energization of said field winding in a direction to return said rotatable element to said neutral position from its position in either direction from said neutral position.

1'7. In combination, a rotatable element for controlling an electrical condition of an electric circuit, said rotatable element having a neutral position intermediate limit control position for either direction of rotation from said neutral p sition, a reversible electric motor for operating said rotatable element in opposite directions from said neutral position, a generator provided with a control field winding and having one polarity or the reverse polarity in dependence upon the direction of energization of said control field winding, a first pair of switching means connected in circuit with one terminal of said field winding for selectively connecting said one terminal for energization in one direction or in the reverse direction in dependence upon the direction of rotation of said rotatable element from said neutral position, a second pair of switching means connected to said other terminal of said control field winding for interrupting the circuit of said control field winding upon operation of said rotata ble element to either one of its limit control positions, means responsive to interruption of current flow in said electric circuit for causing energizatlon of said field Winding in a direction to return said rotatable element to said neutral position, and means controlled by said first pair of switching means for interrupting current flow in said control field winding upon arrival at said neutral position from either direction of travel therefrom.

18. In combination, a pair of alternating current circuits, power conversion apparatus interconnecting said alternating current circuits, means for controlling the power flow through said conversion apparatus, means including a watt responsive device connected to be responsive to the power flow through said conversion apparatus and having a control element for effecting a controlling action in oppositesenses when the power flow in a given direction through said conversion apparatus is above or below a predetermined value, and means for controlling said first mentioned means in accordance with the operation of said control element.

39. In combination, a pair of alternating current circuits, powerconversion apparatus interconnecting said alternating current circuits, means for controlling the power flow through said conversion apparatus, means including a watt responsive device connected to be responsive to the power fiow through said conversion apparatus and having a control element for effecting a controlling action in opposite senses when the power fiow in a given. direction through said conversion apparatus above or below a predetermined value at which said control element is inactive, means for calibrating said watt re sponsive device to establish the predetermined value of power flow at which said control element is inactive, means for changing the setting of said calibrating means, and means for controlling said first mention d mean in accordance with the operation of said control element.

20. In combination, a pair of alternating current circuits, power conversion apparatus interconnecting said alternating current circuits, means for controlling the power flow through said conversion apparatus, means including a watt responsive device connected to be responsive to the power flow through said conversion apparatus and having a control element actuated thereby for effecting 'a controlling action in onpcsite senses when Dower flow in a given direction through said conversion apparatus is above or below a predetermined value at which said control element is inactive, means for calibrating said watt responsive device to establish the predetermined value of power flow at which said control element is inactive, means for indicating the calibration of said watt responsive device, means for changing the setting of said calibrating means in accordance with the indication of said indicating device, and means for controlling 27 said first mentioned means in accordance with the operation of said control element.

21. In combination, a pair of alternating current circuits, reversible power conversion apparatus interconnecting said alternating current circuits, means for controlling the power flow through said conversion app s, means cluding a watt responsive device connected to be responsive to the power flow through said conversion apparatus and having a torque producing element and a control element actuated thereby for effecting a controlling action in opposite senses when the power flow through said conversion apparatus is above or below a predetermined value, means for maintaining the torque of said watt responsive device in the same direction irrespective of the direction of power flow, and means for controlling said first mentioned means in accordance with the operation of said control element above or below said predeter mined value during either direction of power flow.

22. In combination, a pair of alternating current circuits, reversible power conversion apparatus interconnecting said alternating current circuits, means for controlling the power flow through said conversion apparatus, manual means interconnected with said first mentioned means for selecting the direction of power flow through said conversion apparatus and varying said power flow in either direction, automatic means including a wattmeter device connected to be responsive to the power flow through said conversion apparatus above or below a predetermined value for controlling said first mentioned means, and means for transferrin the control of said first mentioned means from said manual means to said automatic means or vice versa.

23. In combination, a pair of alternating current circuits, a pair of power conversion units interconnecting said alternating current circuits, means for controlling the power flow through each of said conversion units, means including a pair of watt responsive devices arranged one with each of said power conversion units and each watt responsive device being connected to be responsive to the power flow through its associated conversion unit, each watt responsive device being provided with a control element for effecting a controlling action in opposite senses when the power flow in a given direction through its associated conversion unit is above or be ow a predetermined value at which said control element is inactive, calibrating means connected to each watt responsive device to establish the predetermined value of power flow at which the control element of its associated device is inactive, adjustable means associated with each of said calibrating means for changin the setting thereof, means for simultaneously adjusting said adjustable means, and means associated with each of said watt responsive devices for controlling said first mentioned means in accordance with the operation of the control element of its associated watt responsive device.

24. In combination, a pair of alternating current circuits, a pair of power conversion units interconnecting said alternating current circuits, means for controlling the power flow through each of said conversion units, means including a pair of watt responsive devices arranged one with each of said power conversion units and each watt responsive device being connected to be responsive to the power flow through its associated conversion unit, each watt responsive device being provided with a control element for effecting a controlling action in opposite senses when the power flow in a given direction through its associated conversion unit is above or below a predetermined value at which said control element is inactive, adjustable means connected to each watt responsive device for adjusting the ratio of power flow of said pair of conversion units, and means associated with each of said watt responsive devices for controlling said first mentioned means in accordance with the operation of the control element of its associated watt responsive device.

25. In an electric power system, a load circuit, a first alternating current system constituting a principal source of energy connected to energize said load circuit, a supplementary source of energy also connected to energize said load circuit, a second alternating current system, a second supplementary source of energy connected to energize said load circuit and comprising a power conversion unit interconnecting said second alterhating current system and said load circuit, means for varying the power flow in either direction through said conversion unit, said principal source and two supplementary sources of energy being connected at a common tie point for jointly energizing said load circuit, an integrating demand measuring instrument connected at said tie point to be energized in accordance with the net interchange of energy between said principal source and said supplementary sources, said measuring instrument being provided with a control element, and means controlled by said control element for controlling said first mentioned means to cause said conversion unit to transmit power from said second alternating current system to said first alternating current system when the energy demand of said load is above said predetermined value and to transmit power in the reverse direction when the energy demand of said load is below said predetermined value.

26. In an electric power system, a load circuit, a first alternating current system of one frequency constituting a principal source of energy connected to energize said load circuit a supplementary source of energy of the same frequency as said first alternating current system connected to energize said load circuit, a second alternating current system of a different frequencythan said first alternating current system, a second supplementary source of energy connected to energize said load circuit and comprising an electronic power conversion unit interconnecting said second alternating current system and said first alternating current system, means for varying the power flow in either direction through said conversion unit, said principal source and said two supplementary sources being connected at a common tie point for jointly energizing said load circuit, an integrating demand measuring instrument connected at said tie point to be energized in accordance with the net interchange of energy between said principal source and said supplementary sources, said measuring instrument being provided with a movable control element constrained to a neutral position for a predetermined value of said net interchange of energy and operated to one or the other of two controlling positions when the net interchange of energy is above or below said predetermined value, and means operative in response to movement of said movable control element to one or the other of its controlling positions for controlling said first mentioned means to cause said converter unit to transmit power from said second alternating current system to said first alternating current system when the energy demand of said load is above said predetermined value and to transmit power in the reverse direction when the energy demand of said load is below said predetermined value.

27. In con-ibination, a pair of alternating current circuits, a pair of parallel connected power conversion units interconnecting said alternating current circuits, means arranged with each of said conversion units for controlling the power fiow through its associated conversion unit, means including a pair of Watt responsive'devices arranged one with each of said power conversion units and each watt responsive device being connected to be responsive to the difference in power transmitted by the respective units of said pair of conversion units, and circuit controlling means for each watt responsive device having an inactive position when th watts of the respective conversion units are in a predetermined ratio and each having one or the other of two control positions for controlling the first mentioned means of its associated conversion unit to increase or decrease the power fiow of its associated conver sion unit in dependence upon the direction of departure of watts in the respective conversion units from said predetermined ratio.

28. In combination, a pair ofalternating current circuits, a first and second power conversion unit connected in parallel relation between said alternating current circuits, means for controlling the power fiow through said first conversion unit, means for controlling the power flow through said second conversion unit, a first watt responsive device including a currengt coil connected to be energized in accordance with the differential current of the respective conversion units and a voltage coil energized in accordance with the voltage of one of said circuits, a second watt responsive device including a current coil connected to be energized in accordance with the differential current of the respective conversion units and a voltage coil energized in accordance with the voltage of said one of said circuits, a movable contact arm arranged to be operated by said first watt responsive device from an inactive circuit controlling position for a predetermined ratio of watts in the respective conversion units to one or the other of two active circuit controlling positions in dependence upon the magnitude and direction of the differential watts of said first watt responsive device, means operative in response to movement of said contact arm to either one of its active circuit controlling positions for controlling said first mentioned means, a movable contact arm arranged to be operated by said second watt responsive device from an inactive circuit controlling position for the said predetermined ratio of watts in the respective conversion units to one or the other or two active circuit controlling positions in dependence upon the magnitude and direction of the differential watts of said second watt responsive device, and means operative in response to movement of the contact arm of said second watt responsive device to either one of its active circuit controlling positions for controlling said second mentioned means.

29. In an electric power system, a load circuit, a first alternating current system constituting a principal source of energy connected to energize said load circuit, a supplementary source of energy also connected to energize said load circuit, a second alternating current system, a

second supplementary source of energy connected toenergize said load circuit and comprising a pair of power conversion units connected in parallel relation between said second alternating current system and said load circuit, means for varying the power fiow in either direction through said conversion units, said principal source and two supplementary sources of energy being connected at a common tie point for jointly energizing said load circuit, an integrating demand measuring instrument having voltage and current coils connected at said tie point to be energized in accordance with the net interchange of energy between said principal source and said supplementary sources, said measuring instrument being provided with a control element, means controlled by said control element for simultaneously controlling said first mentioned means to cause said conversion units jointly to transmit power from said second alternating current system to said first alternating current system when the energy demand of said load is above said predetermined value and to transmit power in the reverse direction when the energy demand of said load is below said predetermined value, and means associated with each of said conversion units and responsive to the ratio of load transmitted by the respective conversion units for modifying said first mentioned means to adjust the loading between said conversion units toa predetermined ratio.

30. In combination, a supply circuit, a load circuit, electric translating apparatus connected between said circuits and including electronic means having an anode, a cathode and a control memher, an excitation circuit comprising a source of alternating current connected to energize said control member, circuit interrupting means connected between one of said circuits and said electric translating apparatus, means responsive to an abnormal operating condition of said electric translating apparatus for causing deenergization of. said control member, and means for causing said circuit interrupting means to disconnect said electric translating apparatus from one of said circuits after a predetermined number of operations of said last mentioned means in a predetermined period of time.

31. In combination, a supply circuit, a load circuit, electronic power conversion apparatus connected between said circuits and comprising electronic means of the type employing an ionizable medium and including an anode, a cathode and an arc-initiating control member, an excitation circuit connected to said control member and comprising a source of alternating current and an inductive device interposed between said source and said control member for transmitting electrical impulses thereto, means for controlling said inductive device to prevent the application of electrical impulses thereto, circuit interrupting means connected between said supply circuit and said power conversion apparatus, means responsive to an abnormal current condition of said power conversion apparatus for causing operation of said first mentioned means, and means for causing said circuit interrupting means to disconnect said supply circuit from said power conversion apparatus after said first mentioned means has performed a predetermined number of its circuit controlling operations in a predetermined period of time.

32. In combination, a pair of alternating current circuits, electronic power conversion apparatus connected between said circuits and comprising two electronic conversion units, each unit mum an are between said anode and cathode, shunting 10 means for rendering said transformer ineffective, a circuit breaker connected between each conversion unit and its associated alternating current circuit and each circuit breaker being provided with a circuit opening control circuit, means rei sponsive to alternating current above a predetermined value transmitted to a conversion unit when operating as a rectifier from its associated alternating current circuit, means responsive to the current of said direct current circuit above a predetermined value, relay means responsive to the operation of either of said two last mentioned means for operating said shunting means, and means for energizing one of said circuit opening control circuits to cause opening of its associated circuit breaker after said shunting means has been operated a predetermined numberof times in a predetermined period of time.

WILLIAM N. GITTINGS; AMOS W. BATEMAN, 

